1. Field of the Invention
The present invention relates to image processing for chroma-correcting an image.
2. Description of the Related Art
Chroma correction processing that corrects the chroma of an image to enhance the brightness of the image and to improve appearance of the image is available.
On the other hand, as a human visual characteristic, a phenomenon called an area effect of colors is known. The area effect of colors is a phenomenon in which one may perceive different lightness and chroma levels of objects depending on their apparent sizes. For example, even when objects have the same color, when one object has a large apparent size in an image, one perceives it to have higher lightness and chroma levels than the other objects with a smaller apparent size. Due to this area effect of colors, a phenomenon in which one perceives different object colors in an image depending on their different visual sizes occurs. For example, if there are regions having the same color but different sizes, one perceives that the larger region has a higher chroma level and is brighter than the smaller region.
As the chroma correction processing, a technique for applying chroma correction to respective pixels of an image according to an amount of chroma correction is available. With this technique, upon uniformly applying chroma correction to an image, a desired effect can be obtained on image data. However, the chroma level changes depending on the apparent sizes of regions due to the area effect of colors, and a desired effect cannot often be obtained. That is, chroma correction must be applied according to the apparent sizes of regions.
Japanese Patent No. 3370770 discloses an invention that corrects a chroma level according to the area of a human skin color in an image. More specifically, that invention detects the area of a skin color region (to be referred to as a skin color area hereinafter) in one frame from color-difference signals, detects the average luminance of the frame from a luminance signal, and determines the amount of skin color correction of that frame based on the average luminance and skin color area. Then, the invention corrects a luminance signal and color-difference signals corresponding to the skin color region according to the amount of skin color correction.
Japanese Patent Laid-Open No. 2004-192614 discloses an invention that corrects a memory color included in image data into a favorable color. More specifically, this invention calculates characteristic values from image data, and extracts a specific color region of a correction target and its background region from the characteristic values and color information of the memory color. The invention calculates a target value of color correction of a favorable color based on the characteristic values, extraction result, and favorable color information, and corrects respective colors of the specific color region based on the target value of color correction and characteristic values. The characteristic values include the average chromaticity of the specific color region, and the occupation ratio of the specific color region to an image area.
However, if there are a large number of small skin color regions in one frame, the technique of Japanese Patent No. 3370770 determines an amount of skin color correction according to the total of areas of these skin color regions, and applies uniform correction to the respective skin color regions. Therefore, this technique cannot appropriately correct each individual skin color region. Also, the technique of Japanese Patent No. 3370770 does not apply color correction to regions other than skin color regions, and the technique of Japanese Patent Laid-Open No. 2004-192614 does not apply color correction to color regions other than the memory color.